It is known that in these cells, the anode reagent is constituted by an alkaline metal, generally sodium, which must be liquid at the operation temperature. The cathode reagent is constituted generally by sulphur and the sodium salts of this substance, but can also be constituted by phosphorus, selenium and the alkaline salts of these substances. In the case where the reagent materials are sulphur and sodium, the electrochemical reaction leads to the reversible formation of sodium polysulphides whose sodium contents increase during discharge. As for the electrolyte, which separates the cathode reagents from the anode reagents, it must be solid at the operation temperature, i.e. at about 300.degree. C., impermeable to the alkaline ions which form in the anode compartment and permeable to electrons. It is generally constituted by sodium beta alumina, i.e. a compound comprising about five to nine molecules of alumina for one molecule of sodium oxide. It generally has the form of a tube closed at its bottom part, containing the anode reagent and immersed in the cathode reagent, the latter reagent being contained in a metal cathode tank and impregnating a graphite felt formed by washers surrounding said tube. The electrolyte tube is held by a support connected in a fluid-tight manner to this anode reagent tank.
Generally, said support is in the form of a plate or disc made of alpha alumina. This disc comprises a central bore in which the beta sodium alumina is set.
In such cells, the discharge process leads to the transformation of the sulphur into sodium polysulphides in the sequence set forth hereinbelow: EQU S.fwdarw.Na.sub.2 S.sub.5 .fwdarw.Na.sub.2 S.sub.4 .fwdarw.Na.sub.2 S.sub.3
during recharging, the opposite process occurs. However, it is observed that it is practically impossible in conventional cells to effect the transformation EQU Na.sub.2 S.sub.5 .fwdarw.S
and consequently to effect complete recharging.
Such a disadvantage results from the fact that during recharging a layer of sulphur is permanently deposited on the electrolyte tube, and acts as a migration barrier with respect to the sodium ions and thus opposes the complete regeneration of the elementary sulphur.
Further, it is observed that the electrochemical reaction within the sulphur is distributed in a not very homogenous manner, this resulting in charging and a recharging which are defective, by a reduction in the capacity and by detrimental ageing phenomena.
To avoid these drawbacks, it has been proposed elsewhere either to displace the sulphur layer in the vapour phase from the electrolyte tube towards the positive current collector, or else to make use of an additive such as a sulphide or arsenic, or else to modulate the conductivity of the body of molten sulphur.
Nonetheless such solutions have not adequately mitigated the said drawbacks. The present applicant has therefore elaborated a new solid electrolyte cell structure suitable for eliminating such disadvantages and for allowing consequently a charging and recharging of the said cell which are practically complete, while avoiding ageing phenomena.